In order to open any DC circuit, the inductive energy stored in the magnetic fields due to the flowing current must be absorbed; it can either be stored in capacitors or dissipated in resistors (arcs that form during opening the circuit are in this sense a special case of a resistor). Because of the rapid inrush of current in a short circuit, the inductive energy can easily be much greater than just the inductive energy stored in the system at normal full load; if the current goes to five times the normal full load amps before being controlled, the inductive energy would be up to twenty-five times as large as in the circuit at normal full load (depending on the location of the short). The inductive energy that must be dissipated to open a high voltage DC (HVDC) transmission line circuit can be in the hundreds of megajoules (MJ). The other major problem with opening a DC circuit is that (unlike AC power), the current and voltage do not go through zero periodically, so it is very difficult to extinguish a DC arc.
Several prior art strategies are known for breaking a high power DC current. Arc chute breakers (U.S. Pat. Nos. 2,270,723; 3,735,074; 7,521,625; 7,541,902 for example) are effective to break DC currents up to 8000 amps (8.0 kA, kiloamps) at 800 volts (0.8 kV, kilovolts) DC, or 4 kA at 1.6 kV. One can go to higher voltage with arc chute breakers, but the needed physical separation of the electrodes and the number of plates in the arc chute increases linearly with voltage in such devices, and so they become very large at voltage higher than 3.5 kV.
The concept behind arc chute breakers is to spread the arc current out into many small arcs over a large surface area between parallel metal plates. Since the arc is quite hot, the higher surface area of the many small arcs implies far greater radiative cooling. As the arcs cool, the resistance goes up so high that the arc current is ultimately quenched; this process takes a while: 50-300 milliseconds (ms) is a typical time between striking the arc and arc extinction in a megawatt (MW) scale arc chute breaker. This long time to open the circuit has little to do with the speed of motion of the electrodes; in a Gerapid™ circuit breaker from GE, for example, the electrodes are separated within 3 ms (milliseconds), but cooling the arc takes up to 100 times as long as that, and the current can continue to increase in case of a short for up to ten ms in an arc chute circuit breaker before it begins to decrease.
Another means known in the prior art to create a high power DC circuit breaker is to use the charging or discharging of a capacitor to momentarily reduce the voltage and current to a level that a fast acting AC-type switch can open the circuit. U.S. Pat. No. 3,809,959 describes an arrangement in which two AC-type switches, a resistor, a spark gap, and a capacitor are combined to give an effective DC circuit breaker that can work up to HVDC voltage. This is faster than an arc chute breaker, and is applicable up to HVDC voltage levels. Later refinements of this idea include pre-charging the capacitor to an opposite polarity compared to the flowing current to be interrupted.
U.S. Pat. No. 3,534,226 describes a particular way to insert resistance and capacitance into a DC circuit, to open the circuit; this patent is included herein by reference in its entirety. The basic concept of switching in resistors to reduce the current in a stepwise manner so as to control the magnitude of voltage transients during opening of a DC circuit is well described in U.S. Pat. No. 3,534,226, which envisions using many individual switches and resistors. The method of U.S. Pat. No. 3,534,226 involves two different kinds of switches that must be opened in a precise sequence: first a low resistance mechanical switch (through which most of the power flows when the circuit breaker is closed) is opened. This is a conventional switch in which the electrical contacts are separated. Although a plasma arc may briefly form between the separating electrodes of the low resistance switch, this arc is quickly extinguished as the current is commutated onto a parallel path through the resistors, which are switched via fast acting switches. The initial resistance in the resistive network must be quite low for the initial arc to extinguish and commutate to the parallel resistive path. By the time the last fast acting switch is opened the current has been reduced to less than 10% of its maximum value (which implies that >99% of the magnetic energy has been dissipated), which allows the final capacitor snubber to be relatively small and economical compared to the size it would have to be if it had to absorb most of the magnetic energy stored in the circuit at the time of initial opening. U.S. Pat. No. 3,534,226 forms the basis for several subsequent patents, including U.S. Pat. Nos. 3,611,031 and 3,660,723 (both of which also use a low-loss mechanical switch to commutate the current to a resistive network based on fast electronic switches), and U.S. Pat. No. 6,075,684 which uses a fast electronic switch in place of the commutating mechanical switch.